The invention relates to a process for the low-temperature fractionation of air using a three-column system. The three-column system has a high-pressure column, a low-pressure column and a medium-pressure column. The medium-pressure column is used to separate a first oxygen-enriched fraction from the high-pressure column, in particular in order to produce nitrogen, which in liquefied form is used a reflux in the low-pressure column or is extracted as product.
The fundamentals of the low-temperature fractionation of air in general are described by the monograph xe2x80x9cTieftemperaturtechnikxe2x80x9d [cryogenics] by Hausen/Linde (2nd edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). In the three-column system, the high-pressure column and low pressure column preferably form a Linde double column, i.e. these two columns are connected so as to exchange heat via a main condenser. (However, in principle the invention can also be applied to other arrangements of high-pressure column and low-pressure column and/or other condenser configurations.) Unlike the conventional Linde two-column process, in the three-column process not all the oxygen-enriched liquid which is formed in the high-pressure column is introduced directly into the low-pressure column, but rather a first oxygen-enriched fraction from the high-pressure column flows into the medium-pressure column, where it is broken down further, specifically under a pressure which is between the operating pressures of high-pressure column and low-pressure column. Top vapour from the medium-pressure column is brought into indirect heat exchange with a cooling fluid and, in the process, is at least partially condensed. Liquid nitrogen which is produced in the process is used as additional reflux in the three-column system and/or obtained as liquid product. For example, it is known to use bottom liquid from the high-pressure column, bottom liquid from the medium-pressure column, an intermediate liquid from the medium-pressure column, bottom liquid from the low-pressure column or an intermediate liquid from the low-pressure column as cooling fluid for condensing top gas in the medium-pressure column. Three-column processes of this type are described, for example, in DE 1065867 B, DE 2903089 A, U.S. Pat. No. 5,692,395 or EP 1043556 A.
In addition to the three abovementioned columns for nitrogen/oxygen separation, further separating devices may also be provided, for example a crude argon column for oxygen/argon separation, a pure argon column for argon/nitrogen separation and/or one or more columns for obtaining krypton and/or xenon, or also non-distillative separating or further cleaning devices. Three-column systems with an additional crude argon column are known, for example, from the abovementioned article by Latimer, from U.S. Pat. No. 4,433,989, EP 147460 A, EP 828123 A or EP 831284 A.
The invention is based on the object of providing a process and an apparatus for the low-temperature fractionation of air using the three-column system which is particularly economically favourable.
This object is achieved by the fact that at least one of the following two process streams is used as cooling fluid of the condensation of the second nitrogen top gas from the medium-pressure column:
a second, liquefied charge air stream, and/or
a liquid from an intermediate point of the high-pressure column.
In this way, the indirect heat exchange in the medium-pressure column condenser can be carried out particularly efficiently.
The first variant of the process according to the invention can be employed in particular for installations with considerable pre-liquefaction of air, i.e. with a high production of liquid and/or a high degree of internal compression. In the case of an internal compression process, at least one of the products (for example nitrogen from the high-pressure column and/or medium-pressure column, oxygen from the medium-pressure column and/or low-pressure column) is removed in liquid form from one of the columns of the three-column system or from a condenser which is connected to one of these columns, is brought to an elevated pressure in the liquid state, is evaporated or (in the case of supercritical pressure) pseudo-evaporated in indirect heat exchange with the second charge air stream and is ultimately obtained as gaseous pressurized product. The air which is liquefied in the process or during a subsequent expansion step is then used as cooling fluid. The evaporated second charge air stream is preferably introduced into the low-pressure column. The liquefied air required (the second charge air stream) may also be produced in liquid installations without internal compression, in an air cycle.
In this context, the terms xe2x80x9cliquefied charge airxe2x80x9d is understood as meaning a stream which has been formed directly by liquefaction of a part stream of the charge air and has not then been subjected to any concentration-changing measure. In particular, no phase separation is performed between liquefaction and introduction into the evaporation space of the medium-pressure column condenser.
The top condenser of the medium-pressure column is preferably designed as a falling-film evaporator. In the process, the cooling fluid is only partially evaporated. The resulting two-phase mixture is introduced into a phase-separation device, in which a fraction which is in vapour form and a proportion which has remained in liquid form are separated from one another. The use of a falling-film evaporator results in a particularly low temperature difference between the liquefaction space and the evaporation space. This property contributes to optimizing the pressure at which the medium-pressure column is operated.
The cooling fluid generally has to be expanded upstream of the indirect heat exchange. Within the context of the invention, this expansion step may be carried out so as to perform work. For this purpose, by way of example, the second charge air stream is introduced, in the liquid or supercritical state, into a liquid turbine, from which it emerges again in completely liquid or substantially completely liquid form.
In many cases, it is expedient to feed a second charge fraction to the medium-pressure column in addition to the first oxygen-enriched fraction which is formed, for example, by bottom liquid from the high-pressure column. For this purpose, an additional fraction, which has a different composition from the first oxygen-enriched fraction, is extracted from the high-pressure column and fed to the medium-pressure column. If an intermediate liquid from the high-pressure column is used as cooling fluid, a part can be branched off and fed to the medium-pressure column of a further charge fraction; the additional fraction and the cooling fluid are in this case extracted from the same intermediate point of the high-pressure column.
The process according to the invention can be carried out without argon being obtained. In the latter case, the medium-pressure column can be heated using any known method, for example by means of condensation of a gaseous nitrogen stream from the high-pressure column, of an intermediate fraction from the high-pressure column or a part stream of the charge air, or else by transferring sensible heat from an oxygen-enriched liquid of the high-pressure column. As an alternative, the bottom heating of the medium-pressure column can be operated with recompressed nitrogen, as explained in detail in an application (German Patent Application (10103957.3 and corresponding applications) which is not a prior publication.
However, the three-column system of the invention can be connected particularly effectively to an argon recovery as a result of a crude argon column, the top vapour from which is condensed in a crude argon condenser, being connected downstream of the three-column system. The crude argon condenser preferably serves at the same time as bottom heating of the medium-pressure column as a result of bottom liquid from the medium-pressure column being at least partially evaporated at that location and oxygen-enriched vapour which is formed in the process being returned to the medium-pressure column. The generation of liquid reflux for the crude argon column and the generation of rising vapour for the medium-pressure column is therefore carried out in a single heat-exchange operation. Therefore, a single condenser/evaporator is sufficient for both functions. This on the one hand leads to a relatively low outlay on equipment, and on the other hand means that the process is particularly favourable in terms of energy on account of the reduction in the exchange losses.
Preferably, the cooling fluid is at least partially evaporated in indirect heat exchange with the second nitrogen top gas from the medium-pressure column, and the fraction in vapour form which is formed in the process is introduced into the low-pressure column, in particular with the aid of a cold fan.
Above the feed for the first oxygen-enriched fraction, the medium-pressure column preferably has mass transfer elements covering at least seven theoretical plates. By way of example, the number of theoretical plates above the feed point is 7 to 50, preferably 16 to 22 theoretical plates. Beneath the feed for the first oxygen-enriched fraction, the medium-pressure column does not have any mass transfer elements, or has mass transfer elements amounting to 1 to 5 theoretical plates.
The invention also relates to an apparatus for obtaining argon.
The invention and further details of the invention are explained in more detail below with reference to exemplary embodiments illustrated in the drawings.